With a lively group of kindergartners, McMillan introduces the concept of comparison, with examples like yellow, yellower and yellowest or close, closer and closest. The children build a tower of blocks to demonstrate the various degrees of tallness. A single shouting girl is joined by her yelling friends to exhibit ``loud, louder and loudest.'' McMillan's photographs present the world of kindergarten clearly and realistically, capturing moments that are funny, interesting or universal. At the end, the author offers details about his subjects and techniques, rounding out this superlative book about comparatives. Ages 3-6. (Apr.)